The present invention relates generally to the production of steel plate for structural applications, and more specifically to the manufacture of as-hot-rolled, high strength, low alloy steel plate which, in the as-hot-rolled condition, is characterized by a high yield strength of at least 80 ksi and by a combination of excellent toughness, formability and weldability.
The development of as-hot-rolled plate steels as alternatives to more expensive heat treated steels for structural applications has been of long-standing interest to the steel industry. This interest has been heightened by the increasing costs of energy required in heat treatment operations. Progress has been made in the development of tough, weldable, microalloyed controlled-rolled plate products as substitutes for normalized steels in applications requiring yield strengths that do not exceed about 70 ksi. Less progress has been made in providing as-rolled plate for higher strength applications, e.g. applications requiring yield strengths in excess of 80 ksi where quenched and tempered steels now predominate.
The reasons for this are largely attributable to the severe metallurgical restraints in achieving high strength by air cooling directly off the plate mill while meeting the weldability, toughness and formability requirements essential for structural applications. Factors which are known to be important in controlling the strength, weldability, toughness and formability of structural steel plate include a ferrite-pearlite microstructure, the proportion of pearlite in the microstructure, grain refinement, precipitation hardening by carbides and nitrides, and a restricted carbon equivalence. These factors often conflict with each other or act at cross-purposes. For example, when the carbon content is maintained at low levels below about 0.2% in order to promote weldability, formability and and impact properties, it may not be possible to achieve an 80 ksi yield strength even when the steel is microalloyed with columbium and/or vanadium. As the carbon equivalence level is increased to improve the strength, the toughness, weldability and formability are reduced. Controlled-rolling, while known to be beneficial from the standpoints of ferrite-grain refinement and consequent toughness and formability, tends to raise the Ar.sub.3 temperature and adversely affect vanadium-columbium strengthening.
The ability of low carbon ferrite-pearlite steels microalloyed with columbium and/or vanadium to meet 70 ksi minimum yield strength requirements with good toughness and weldability has been established on a commercial basis. Their production emphasizes controlled rolling for grain refinement together with low carbon levels for improved impact properties and weldability. A major difficulty arises in achieving 80 ksi yield strengths in air cooled, low carbon ferrite-pearlite steels because of the limitations in the precipitation hardening potential of vanadium and columbium. The strengthening increase produced by columbium and vanadium diminishes with increasing levels of these elements so that yield strengths of about 75 ksi are difficult to obtain despite microalloying to uneconomical levels. It is known that the major precipitation strengthening mechanism in air-cooled, ferrite-pearlite steels is largely associated with the precipitation of microalloyed carbonitrides during transformation and that the degree of precipitation strengthening is inversely related to the ferrite transformation temperature which controls the dispersion and size of the precipitates. The very factors which provide for excellent toughness, namely, low carbon content and controlled-rolling, combine to raise the ferrite transformation temperature and thereby severely limit the degree to which precipitation strengthening is obtained.
In attempting to achieve high strength combined with good toughness, formability and weldability, it has been proposed to use an aluminum-killed steel containing 0.02-0.26% carbon, 1.25-1.75% manganese, 0.75-1.5% silicon, 0.003-0.015% nitrogen, 0-0.07% vanadium, and 0-0.03% columbium, and to subject the steel to a hot rolling finishing operation in which there is at least a 25% hot rolling deformation above the A.sub.1 temperature. In this procedure, the controlled-rolling is mainly for the purpose of grain refinement. With carbon contents ranging from about 0.18-0.26%, it was suggested that the finishing operation could be carried out in the two-phase region in which the microstructure is both austenite and ferrite. It was also suggested that the cooling procedure can be conducted in still air or with a water spray or with air impingement. The yield strengths attributed to higher carbon level steels (0.18-0.26%) processed in the manner described range from 70 to 85 ksi. The optional use of columbium and vanadium in a maximum amount of 0.07% indicates that the microalloying strengthening mechanism is primarily due to grain refinement and that water cooling and maximum carbon content are required to achieve yield strengths in excess of 80 ksi. At the higher carbon levels required for optimum strength, careful control must be exercised when cooling by the water spray and air impingement methods in order to avoid the formation of bainite.
A related prior art practice is described in U.S. Pat. No. 4,008,103 to Miyoshi et al. The process is characterized by low temperature austenitization, e.g., 800.degree. to 950.degree. C., followed by fishing rolling in the two-phase or alpha-gamma region well below the Ar.sub.3 temperature. Intercritical rolling is an essential feature of the invention and is necessary to achieve minimum yield strengths of 80 ksi.
Although rolling in the two phase region can produce yield strengths in excess of 80 ksi, this approach has significant disadvantages. The finishing requirements are severe and are beyond the practical operating limitations of some commercial mills. Other drawbacks include high mill loads, long rolling times, and shape and flatness control problems. Rolling in the two phase region also can produce properties which are anisotropic in the plane of the plate so that it may delaminate or split. In addition, the Charpy V-notch impact properties in the plane of the plate may be adversely affected.
An alternative prior art approach uses a killed low-alloy steel containing 0.12-0.20% carbon, 1.10-1.65% manganese, 0.05-0.20 vanadium, 0.005-0.025% nitrogen and 0.60% maximum silicon. In order to possess the desired properties of good toughness, weldability and formability combined with a yield strength in excess of 80 ksi, this steel is hot finished in a temperature range of 1550.degree.-1650.degree. F., cooled at a rate of from 20.degree.-135.degree. F. per second, and collected, as by coiling or piling, within a temperature range of from 1025.degree.-1175.degree. F. The steel must be water quenched to achieve the cooling rate that is necessary to obtain yield strengths of at least 80 ksi and must be collected above a minimum temperature (1025.degree. F.) to avoid the formation of lower transformation products, e.g., bainite, in the microstructure.